Hybrid vehicles with parallel hybrid powertrains of the state of the art usually feature, arranged between an internal combustion engine and the output drive of the vehicle, a starting element in the form of a friction clutch in order to start the vehicles with the internal combustion engine in a commonly known manner, wherein the starting element is first operated in a slip mode during the starting procedure. With the hybrid vehicle featuring only a starting procedure using the onboard electric motor, the open loop and closed loop controlled slip mode operation of the starting element is not necessary, because, in contrast to conventional internal combustion engines, the electric motor does not have a minimum rotational speed.
A method is described in DE 10 2004 02 061 for controlling an open loop and closed loop of a powertrain of a hybrid vehicle and a powertrain of a hybrid vehicle equipped with a friction-locking starting element. With this inventive method for open loop and closed loop control of a powertrain of a hybrid vehicle with a internal combustion engine, an electric motor, a shifting element, arranged between the electric motor and an output within a power flow of the powertrain and designed with continuously variable transmission capacity, and a clutch arrangement arranged between the electric motor and the internal combustion engine, via which the electric motor and the internal combustion engine can be functionally connected, a powertrain of a hybrid vehicle can be operated in such a way that switching over from driving the hybrid vehicle by means of the electric motor to a parallel drive mode of the hybrid vehicle facilitated by the electric motor and the internal combustion engine or to driving the hybrid vehicle solely by means of the internal combustion engine as well as starting the internal combustion engine through the electric motor can be performed without any noticeable powertrain reaction torque detectable to the hybrid vehicle driver.
When this powertrain operates, the power transmission capacity of the shifting element is adjusted during a startup procedure in such a manner that a torque is present, which is independent of the startup procedure of the internal combustion engine at the output of the powertrain, where changes in torque at the output appearing as a result of the starting internal combustion engine are prevented preferably through a slip mode operation of the shifting element.
In this method, the rotational speed of the electric motor during the start of the internal combustion engine is increased to a rotational speed value, wherein it is ensured that the shifting element between the electric motor and the output of the powertrain is held in a slip mode operation during the entire internal combustion engine startup procedure. The rotational speed value is calculated via an algorithm implemented in the engine control, the transmission control and/or a subordinate memory manager.
The prior art also includes vehicles with internal combustion engines featuring their own minimum rotational speed, where the engines are provided with a starting element in the form of a hydrodynamic torque converter to allow the vehicle to be started by the internal combustion engine.
It is the object of invention to present a method for operating a parallel hybrid powertrain of a vehicle that is executed in the area between the electric motor and the output with a hydrodynamic torque converter and at least one additional friction-locking shifting element, the application of which allows a parallel hybrid powertrain to be operated over its entire operating range in a scope that ensures a high degree of driving comfort.